GLENN RESEARCH CENTER, Ohio — If you are wanting to perform some science at Neptune, or Pluto, or beyond in the dark depths of the outer solar system, your spacecraft is going to need power for a very long time, says Spaceflight Insider.

Engineers at NASA’s Glenn Research Center in Cleveland, Ohio, are working to make that happen, and have been at it for a very long time.

The engineering team in NASA Glenn’s Thermal Energy Conversion Branch recently set a run-time record for a free-piston Stirling engine at full power.

The experimental unit, designated Technology Demonstration Converter (TDC) #13, has now performed more than 110,000 hours of cumulative operation, since 2003. That is more than 12 years of operation. Accomplishing this record makes TDC #13 the longest-running heat engine in the history of civilization. And it is still running today, without any issue or any sign of wear.

“It is executing the free-piston Stirling cycle,” Sal Oriti, engineer with the Dynamic Radioisotope Power Systems Project, told Spaceflight Insider. “No contact between the moving parts. Wear mechanisms have been eliminated by non-contacting bearings and non-contacting seals. And with the specialized engineering of the components, most notably the hot end material, you can build something that lasts a very long time.”

Between 2001 and 2003, a number of Stirling converter prototypes were fabricated at NASA Glenn and placed on extended operation tests to demonstrate long-duration life. TDC #13 and TDC #14 have run the longest, with TDC #13 as the record-setter and TDC #14 not far behind.

The two units have been turned off during the experiment only long enough to remove covers to examine their components, and to do the necessary maintenance and replacement of outside equipment and sensors used to conduct and monitor the continuing test. TDC #15 and TDC #16 are also not far behind their slightly older Stirling iterations.

The free-piston Stirling cycle engine is an elegantly simple design. It consists of a pressure vessel that contains a piston and a small quantity of helium gas. When one end of the pressure vessel is heated (in the case of a spacecraft application that heat source will be a piece of the radioisotope plutonium-238), the helium gas within the pressure vessel expands, forcing the piston away and into an oscillating back-and-forth cycle.

A rod attached to the bottom of the piston contains a magnet. As this magnet oscillates in relation to an electromagnetic coil, its oscillation induces a flow of electric current and thus serves as a linear alternator.

All of the motion is done in a back and forth, or linear, fashion. There is no rotary motion. No crank apparatus. The moving parts move only back and forth. And more importantly, they never touch anything. There is no friction.

This is quite unlike the heat engines in our automobiles. They have piston rings to help create a seal in the cylinder, which causes friction. So oil is introduced to reduce the friction. The moving parts of the piston cranks all experience friction as well. So they too must be lubricated and so on.

“All of that just kind of kicks the can down the road for failure,” Oriti said. “It can be done, and you can get about 5,000 hours out of that engine. But you have wear mechanisms and you can’t design an engine like that to last forever. If you want long life, on the order of ten years or twenty years continuous operation, then you have to eliminate all mechanisms of wear.”